1. Field of the Invention
The present invention generally relates to chemically amplified negative tone photoresists based on poly(hydroxystyrene) (PHS) and, more particularly, to compositions and methods for allowing sub-micron patterns to be reliably made in PHS based photoresists.
2. Background Description
Manufacturing of semiconductor devices is dependent upon the accurate replication of computer aided design (CAD) generated patterns onto the surface of a device substrate. The replication process is typically performed using lithographic processes followed by a variety of subtractive (etch) and additive (deposition) processes. Optical lithography patterning involves projecting an image of a pattem, reduced in size, onto a photosensitive film (photoresist or resist) on the device substrate. Other techniques may also be employed whereby radiant energy is projected onto the photoresist to effect relative changes (chemical or physical) between the exposed and unexposed portions of the photoresist material, such that portions of the photoresist material can be selectively removed to leave a desired photoresist pattern on the substrate surface. Negative tone photoresists are those types of resists wherein the portions of photoresist exposed to radiation remain on the substrate (e.g., chip, wafer, etc.) and the unexposed portions are removed from the substrate by developer. Positive tone photoresists perform oppositely, whereby the exposed portions are removed by developer, and the unexposed portions remain on the substrate.
Chemically amplified photoresists based on PHS are a class of photoresist that are patterned by using radiant energy to selectively activate a photo-acid compound dispersed within the polymer resin, whereby the photo-acid compound generates an acid which acts locally to either cause crosslinking of polymer chains by initiating crosslinking between the chains with a crosslinking agent distributed within the resin, or by removing acid labile moieties bonded to the phenolic hydroxy moities of the PHS polymer. Chemical amplification of photoresist materials are described in detail in U.S. Pat. No. 5,252,435 to Tani et al., U.S. Pat. No. 5,258,257 to Sinta et al., U.S. Pat. No. 5,352,564 to Takeda et al., and U.S. Pat. No. 5,210,000 to Thackery et al., all of which are herein incorporated by reference.
Chemically amplified negative tone photoresists have the inherent advantages of thermal stability and very high photospeeds. This allows for higher stepper thoughput and can eliminate the need for ultraviolet (UV) hardening.
Recently, researchers have been investigating the possibility of using chemically amplified negative tone photoresists in lithography applications having sub-micron resolution requirements (see, for example, U.S. Pat. No. 5,296,332 to Sachdev et al., which is herein incorporated by reference). Thackery et al., Proc. SPIE, 2195:152 (1994) describes a phenomenon described as "microbridging" which occurs with PHS based resins. Microbridging is the spontaneous formation of insoluble residues in unexposed regions. Thus, upon development, resist may remain between nested arrays or in other areas where it should have been removed. These fine bridges between high resolution resist patterns will adversely impact on the ability to precisely pattern the substrates. Microbridging is a problem only in sub-micron lithography applications (e.g., when the resolution requirements are on the order of 1-3 .mu.m, microbridging is not a significant problem, but when the resolution requirements on the order of 0.2-0.5 .mu.m, microbridging becomes an important problem).
Resist formulations based on novolak resins do not form microbridges. However, novolak resins are not viable resin canditates for high performance chemically amplified negative tone resists. Novolaks do not produce vertical profiles, as is required in high resolution lithography. In addition, Novolaks have too slow a dissolution rate in 2.38 wt % trimethyl ammonium hydroxide (TMAH), which is now the industry standard developer. Furthermore, the thermal stability of novolaks is significantly less than PHS (e.g., 130.degree. C. vs. 200.degree. C.).
Thackery et al., Proc. SPIE, 2195:152 (1994), demonstrated that microbridging problems could be reduced, but not eliminated, by using low molecular weight (e.g., &lt;3800 daltons) PHS and 1.2 wt % TMAH developer. However, the approach of Thackery et al. is not well suited for industrial applications. Specifically, it does not allow using PHS having a wide range of molecular weights, thus, it places significant restrictions on the manufacture of the photoresist composition. In addition, 1.2% TMAH is a 50% reduction from the industry standard aqueous developer of 2.38% TMAH, hence, additional formulation and processing requirements are placed on the manufacturer. Furthermore, the use of a lower concentration of TMAH will increase the time required for development, and may increase the total volume of developer required if a spray operation is used for development. Both effects will increase the cost of producing the patterned substrates. Moreover, the Thackery et al. method was not completely effective and did not allow the production of lines which are less than 0.35 .mu.m wide via I-line procedures.